Mass spectrometry is an analytical methodology often used for quantitative elemental analysis of materials and mixtures of materials. In mass spectrometry, a sample of a material to be analyzed called an analyte is broken into particles of its constituent parts. The particles are typically molecular in size. Once produced, the analyte particles (ions) are separated by the spectrometer based on their respective masses. The separated particles are then detected and a “mass spectrum” of the material is produced. The mass spectrum is analogous to a fingerprint of the sample material being analyzed. The mass spectrum provides information about the masses and in some cases quantities of the various analyte particles that make up the sample. In particular, mass spectrometry can be used to determine the molecular weights of molecules and molecular fragments within an analyte. Additionally, mass spectrometry can identify components within the analyte based on the fragmentation pattern when the material is broken into particles (fragments). Mass spectrometry has proven to be a very powerful analytical tool in material science, chemistry and biology along with a number of other related fields.
A specific type of mass spectrometer is the time-of-flight (TOF) mass spectrometer. The TOF mass spectrometer (TOFMS) uses the differences in the time of flight or transit time through the spectrometer to separate and identify the analyte constituent parts. In the basic TOF mass spectrometer, particles of the analyte are produced and ionized by an ion source. The analyte ions are then introduced into an ion accelerator that subjects the ions to an electric field. The electric field accelerates the analyte ions and launches them into a drift tube or drift region. After being accelerated, the analyte ions are allowed to drift in the absence of the accelerating electric field until they strike an ion detector at the end of the drift region. The drift velocity of a given analyte ion is a function of both the mass and the charge of the ion. Therefore, if the analyte ions are produced having the same charge, ions of different masses will have different drift velocities upon exiting the accelerator and, in turn, will arrive at the detector at different points in time. The differential transit time or differential ‘time-of-flight’ separates the analyte ions by mass and enables the detection of the individual analyte particle types present in the sample.
When an analyte ion strikes the detector, the detector generates a signal. The time at which the signal is generated by the detector can be used to determine the mass of the particle striking it. In addition, for many detector types, the strength of the signal produced by the detector is proportional to the quantity of the ions striking it at a given point in time. Therefore, for these detector types, the quantity of particles of a given mass often can be determined as well as the time of arrival. With this information pertaining to particle mass and quantity, a mass spectrum can be computed and the composition of the analyte can be inferred.
Of significant importance to the performance of a TOF mass spectrometer is the design and performance of the ion detector. Ideally, the detector should have high sensitivity, low noise and high dynamic range. In addition, the detector should provide good temporal resolution. Sensitivity is a measure of the ability of the detector to register the presence of particles arriving individually. An ideal detector would be able to register the arrival of a single ion of any mass and arbitrary energy. However, in practice, detectors often require a number of ions arriving simultaneously to produce a measurable response or signal. High sensitivity refers to the ability of a detector to produce a measurable signal from the impact of a single or very small number of ions. Dynamic range, on the other hand, is a measure of the ability of the detector to produce a signal that is proportional to the number of particles striking the detector at a given point in time. High dynamic range refers to the situation when there are a very large number of particles striking the detector and the detector is still able to produce a signal that is proportional to the number of particles. Temporal resolution refers to the ability of a detector to distinguish between particles based on time of arrival. The arrival of a particle at a detector is often referred to as an “event”. If two events occur at times that are less than the time resolution of the detector, the particles will be indistinguishable and will be registered by the detector as having the same mass. Therefore, time resolution afforded by a detector determines the mass resolution of the TOF mass spectrometer.
A number of different detector types are used in TOF mass spectrometers. Among these are the channeltron, Daly detector, electron multiplier, Faraday cup, and microchannel plate (MCP). The channeltron is a horn-shaped continuous dynode. The inside of the channeltron is coated with an electron emissive material such that when an ion strikes the channeltron it creates secondary electrons. These secondary electrons create more electrons in an avalanche effect and are ultimately detected as a current pulse at the output of the channeltron. The Daly detector is made up of a metal knob that produces secondary electron emissions when struck by an ion. The secondary electrons are accelerated in the Daly detector and, in turn, strike a scintillator that produces photons. The photons are detected as light by a photomultiplier tube (PMT) that then produces the output signal of the detector indicating the presence of an ion impact. An electron multiplier (EM) is similar to a photomultiplier and consists of a series of biased dynodes that emit secondary electrons when the first dynode is struck by an ion. A Faraday cup is a metal cup placed in the path of the ion beam. The cup is connected to an electrometer that measures the ion-current of the beam. The microchannel plate (MCP) is an array of glass capillaries the inside surfaces of which are coated with an electron-emissive material. The capillaries, which typically have an inner diameter of 10–25 um, are biased at high voltage so that when an ion strikes the electron-emissive coating, an avalanche of secondary electrons is produced. The secondary electron avalanche cascade effect creates a gain of between 103 and 104 and ultimately produces an output current pulse corresponding to the initial ion impact event.
FIG. 1 illustrates a typical MCP 10 detector configuration along with an expanded close-up cross-section 18 of a single channel within the MCP. The MCP 10 is positioned in front of an anode plate 11 such that the analyte ions 12 strike the MCP 10 instead of the anode plate 11. An analyte ion 12 that enters a channel 14 eventually strikes the sidewall 15 of the channel 14 within the MCP 10. The sidewall 15 is coated with an electron emissive material. The impact of the analyte ion 12 on the electron-emissive material coating the sidewall 15 causes the emission of secondary electrons 16. The secondary electrons 16 created by the impact of the analyte ion 12 radiate from their point of creation and often impact the sidewalls 15 of the channel 14, for example, as illustrated in FIG. 1. Each impact of secondary electrons 16 with a sidewall 15 can result in the creation of more secondary electrons 16. The end result is that one analyte ion 12 results in the creation of a large number of secondary electrons 16 that ultimately exit the MCP 10 and strike the anode plate 11, often a Faraday cup, where they can be detected as a current pulse. The total number of secondary electrons exiting the MCP and striking the anode plate 11 that are produced by the impact of a single analyte ion 12 is often called the detection gain of the MCP 10. The MCP 10 in this configuration functions as an electron multiplier (EM).
The number of secondary electrons 16 produced by the MCP 10 is proportional to the length of the channels 14 in the MCP 10. A longer channel 14, in principle, will result in more impacts and thus, the production of more secondary electrons 16. However, there is a practical limit to the detection gain of a given MCP 10. Once a sufficient number of secondary electrons 16 has been produced, further production of secondary electrons 16 is inhibited by the current or electric field associated with the secondary electrons already produced. This phenomenon results in saturation of the detector. Saturation limits the achievable gain in the MCP 10 detector. In addition, electrons under high concentration conditions can cause positive ions to be formed which travel backward in the channel. The backward motion known as “feedback” hurries the onset of saturation and can cause the creation of ghost peaks or artifacts in the detected output. Similar saturation limits and ghost peaks are observed in the other detector types as well when these detectors are designed simultaneously for high gain, high sensitivity and high dynamic range.
Recently, hybrid electron multiplier detectors have been developed to improve the gain and reduced or overcome the saturation limits, and to increase the dynamic range of the above-described detectors without introducing artifacts. Typically, these hybrid detectors have been created by cascading two of the above referenced multiplier types. The objective of these hybrid combinations is to overcome the above-described inherent limitations of non-hybrid detectors in terms of the detection sensitivity, gain, dynamic range and resolution of very fast and/or short-lived input events that represent the data of interest in TOF measurements, as in TOF mass spectrometry (TOFMS).
One example of such a hybrid detector, known as a Chevron configuration, is illustrated in FIG. 2a. In the Chevron configuration hybrid detector 20, a second MCP 21 is placed between the first MCP 10 and the anode plate 11. The first MCP 10 in the Chevron configuration hybrid detector 20 of FIG. 2a, like the MCP 10 of FIG. 1, provides a large, flat detection surface to the incoming ions or ion packets. These ions are detected synchronously in time, thereby providing this hybrid detector 20 with high sensitivity. However, in the Chevron configuration, the second MCP 21 provides additional gain beyond that produced by the first MCP 10 since the second MCP 21 intercepts the secondary electrons produced by the first MCP 10 and produces even more secondary electrons. Furthermore, unlike the case of lengthening the channels to increase gain, the use of a second MCP 21 allows for greater dynamic range through a delay in the onset of saturation. The delay in the onset of saturation is produced by careful, independent design of the individual MCPs 10, 21 and through independently setting the bias levels of the pair of MCPs 10, 21. In principle, the first MCP 10 is designed and biased for high sensitivity and the second MCP 21 is designed and biased for high saturation. Thus, by cascading two MCPs 10, 21 in the Chevron configuration, the gain of the overall detector 20 is improved and the saturation level is increased compared that of a single MCP 10 design. The Chevron configuration of MCPs 10,21 has been shown to achieve detection gains of 106 to 108.
Unfortunately, even though the two MCPs 10, 21 of the Chevron configuration can be designed and biased independently, this type of hybrid detector 20 still suffers from relatively severe limitation in gain due to saturation, which limits the useful gain of this type of hybrid detector. Further, the Chevron configuration has low dynamic range due to the inherently high resistance of the MCP plates. The high resistance limits the secondary electron production once large numbers of electrons are present, which is particularly evident in and problematic for the second MCP 21. Additionally, ghost peaks or artifacts due to ion feedback can still be produced.
A second approach to hybrid detector design is a hybrid detector 25 comprised of a combination of an MCP and a discrete dynode electron multiplier (DEM) 24 as illustrated in FIG. 2b. In this detector configuration 25, the secondary electrons output by the MCP 10 act as an input to the DEM 24. The DEM 24, in turn, provides further amplification of the detection signal by producing more secondary electrons from those output by the MCP 10. Unlike the MCPs 10, 21, the DEM 24 is capable of supporting large peak signal currents while maintaining linearity. That is, the DEM 24 is much less susceptible to saturation than the second MCP 21 of the Chevron configuration 20 of FIG. 2a. Thus, the first MCP 10 in this hybrid detector provides the desired high sensitivity while the DEM 24 produces additional gain and supports high currents necessary for high dynamic range.
Unfortunately, the DEM 24 has an inherent path-length difference for various ions and electrons. This path-length difference results in a widening of the output signal pulse, Δt, and the generation of spurious trailing pulses or peaks referred to as ghosts peaks or artifacts. The widening of the output signal pulse Δt and presence of spurious trailing pulses reduce the temporal resolution of the detector 25 and limits the useful dynamic range and resolution this type of hybrid detector 25.
The term “Δt” as used herein refers to the widening in time of the output secondary electron signal pulse after the impact of the analyte ion or input electron. For optimum performance, the detector should have a minimum Δt. In particular, for TOFMS, the minimization of the Δt of secondary electrons created from incoming primary analyte ions is very desirable. The Δt is ultimately related to the temporal resolution of the detector.
Conventional electron multipliers (EMs) used for hybrid detectors, such as the classic DEM, are not optimized for this low Δt requirement. For example, one of the best discrete DEMs has a dynode resembling a “venetian blind”. In this particular EM, the ion-to-electron conversion or electron to secondary electron amplification takes place in an “in-line” manner as the electron avalanche proceeds down the length of the DEM structure. While this venetian-blind style dynode provides high sensitivity and dynamic range, the DEM exhibits a rather large Δt. The Δt in the “Venetian Blind” DEM is typically longer than 10 to 20 nanoseconds, which effectively sets the minimum temporal resolution or peak width of this detector type. Modern TOF mass spectrometry generally requires much better resolution than 10 to 20 nanoseconds.
Thus, it would be advantageous to have a hybrid detector with an EM that did not generate spurious trailing pulses or ghost peaks and artifacts, was not susceptible to high level saturation, and did not have inherent path length differences that result in loss of time resolution due to unacceptably high Δt. Such a hybrid detector would provide significant improvement to TOF mass spectrometry and solve a longstanding problem in the art.